This study will be primarily concerned with the isolation and characterization of the following heterophile antigens: infectious mononucleosis heterophile antigen (Paul-Bunnell antigen, PBA), Hanganutziu--Deicher antigens, and heterophile transplantation antigens. These antigens are detected on bovine erythrocyte membranes and can be extracted in soluble form. PBA activity is found in the minor glycoprotein fraction of bovine erythrocyte membranes and has lipid-like properties. PBA will be further purified and its composition determined. Studies on amino acid sequence and oligosaccharide sequence and linkage will be carried out. PBA-like substances will also be studied on normal and malignant human lymphoid cells. Hanganutziu-Deicher antibodies (or "serum sickness antibodies") are apparently directed toward the ganglioside components of bovine erythrocyte membranes. The specificity of the reaction will be examined and the immunodeterminants defined. The neutral glycolipid fractions of bovine erythrocyte membranes contain substances that are reactive with heterophile transplantation antibodies. The identity of these substances will be established and their structures determined.